Final answer:
The length of time it takes for someone to mentally check out from a lecture can vary, but studies suggest it is typically around 20-30 minutes.
Step-by-step explanation:
From the information provided, it is difficult to determine an exact timeframe for when someone might 'check out' mentally during a lecture. However, various studies suggest that attention span during a lecture can vary from person to person and depends on factors such as the topic, delivery style, and individual preferences. In a typical scenario, it is commonly believed that students may start losing focus after around 20-30 minutes of continuous listening.